A resistive random access memory (RRAM) operates according to a characteristic of a resistance changing material, such as a transition between a metal and an oxide. In a resistance changing material, the resistance may change according to an applied voltage. For example, when a voltage equal to or higher than a set voltage is applied to a resistance changing material, the resistance of the material may decrease. This state of decreased resistance is referred to as an ON state. When a voltage equal to or higher than a reset voltage is applied to the resistance changing material, the resistance of the material may increases. This state of increased resistance is referred to as an OFF state.
Conventional resistive random access memory (RRAM) devices may include, for example, a nickel oxide (NiOx) layer as a resistance change layer in the storage node.
FIG. 1 is a graph showing a current-voltage characteristic of a conventional RRAM. Referring to FIG. 1, the horizontal width of area A of the conventional RRAM has a wide range of voltage in which a resistance status starts to change.
It is desirable that a resistance change layer should have the same resistance status at the same applied voltage, which is not the case for conventional RRAMs. That is, when the distribution of voltages that cause a resistance change is too broad, it may be difficult to detect a change of resistance of the resistance change layer in a limited range of voltage. Thus, the reliability of the data read from the conventional RRAM may be low.
In addition, the resistance change layers of conventional RRAMs may be formed by a reactive sputtering method in which O2 gas is often used as a reactive gas. However, such a reactive sputtering method is complicated, and the resistance change layer formed by the reactive sputtering method may have low reproducibility characteristics.